1. Field of the Invention
The present invention generally relates to thermal transfer type printers, and more particularly to a thermal transfer type printer in which optimum heating value for each heating element is automatically selected in accordance with the kind of printing paper.
2. Prior Art
FIG. 1 shows a diagrammatic constitution of a conventional thermal transfer type printer. In FIG. 1, 1 designates a supply bobbin for being wound by a transfer film 2 and for rotating in a direction A so as to feed forward the transfer film 2, 3 designates a take-up bobbin for rotating in a direction B so as to take up the used transfer film 2, and 4 designates a take-up motor for driving the take-up bobbin 3 to rotate. In addition, 5 designates a main sprocket pin feed wheel (hereinafter, referred to as a platen roller) which rotates in a direction C so as to draw in and feed forward a printing paper P in a direction D, and 6 designates an auxiliary sprocket pin feed wheel (hereinafter, referred to as an auxiliary wheel) for rotating in a direction E. Furthermore, 15 designates a thermal head arranged by a plurality of heating elements along the line, and the thermal head 15 is supported by a shaft (not shown) so that the thermal head 15 can move in a radius direction. When the platen roller 5 is forced to move toward the shaft direction, the transfer film 2 and the printing paper P are lapped and pressurized between an outer surface of the platen roller 5 and the heating portion provided with the heating elements within the thermal head 15.
In the above state, the heating elements within the thermal head 15 are heated in accordance with an arbitrary heating pattern, hence, thermal melting ink painted on the transfer film 2 is melted and transferred on the printing paper P in accordance with the arbitrary heating pattern. Thus, the printing operation of first line is performed. The rotating force of drive motor (not shown) is transmitted to the platen roller 5 via a belt 7 and a belt pulley 8 so that the platen roller 5 rotates in the direction C. The rotating force of the platen roller 5 is transmitted to the auxiliary wheel 6 via a belt pulley 9 and a belt 10 so that the auxiliary wheel 6 rotates in the direction E. Hence, the printing paper P is fed forward by a one line distance. At the same time, the take-up bobbin 3 is rotated by the take-up motor 4 in the direction B so that the transfer film 2 is taken up by one line distance. Similarly, a transfer operation of one line and a paper feeding operation of one line is repeatedly performed in turn so that the printing operations of second, third and other lines will be sequentially performed.
In the above-mentioned thermal transfer type printer, a density setting switch is provided on a operation panel, and an optimum printing density can be arbitrarily selected only by operating the density setting switch. In order to obtain a high printing density, the heating value of each heating element must be enlarged by enlarging the value of the current supplied to each heating element or by enlarging a current-on time when the current flows through the heating element. Therefore, the current-on time for the each heating element within the thermal head 15 is selected to an optimum current-on time by use of the density setting switch so that the printing density will be set to an optimum printing density.
Meanwhile, the ordinary paper and a transparent sheet for an overhead projector (OHP) can be applied to the conventional thermal transfer type printer. In the case where the image is printed on the transparent paper for OHP, the heating value of each heating element within the thermal head 15 must be set lower than that in the case where the image is printed on the ordinary paper. Compared to the ordinary paper, the transparent sheet for OHP has lower thermal conductivity and higher smoothness of the surface thereof. When the thermal transfer printing is performed at a high temperature, the bleeding will be generated and the transfer film 2 will be transformed because of the high temperature. Hence, a good printing quality can not be obtained in some cases.
Therefore, in the case where the thermal transfer printing is performed on the transparent sheet for OHP, low printing density must be selected by the density setting switch and the heating value of each heating element within the thermal head 15 will be set to a relatively low heating value. However, the density setting switch must be operated at every time when the ordinary paper is exchanged for the transparent sheet and when the transparent sheet is exchanged for the ordinary paper. Hence, the conventional printer is disadvantageous in that it is complicated and troublesome for the operator to select the optimum density by use of the density setting switch. In addition, the conventional printer is disadvantageous in that there is a possibility in which the printing paper will be printed with undesirable printing density when the density setting switch is operated and the undesirable printing density is selected by mistake.